So, just for fun, I searched/looked up 'check for variables with undefined values' in RenPy, but PyTom (all hail the great god of RenPy PyTom) seemed to indicate in a post that there wasn't an easy way to do this. Further searching simply pointed out that you should define variables in the .init block, etc. which won't help for previous saves, as those saves have 'outdated' variable lists, and variable values in LR are often assigned within the event, with new variables being introduced as new events are added to the game.
Looking through some of the other posts on LemmaSoft, I didn't find a better answer. Essentially, you are supposed to define your variable before you do any checks on said variable. LR events often check variables to see if you've done an event or not, and previous saves won't have a 'default value' for any such events added in later versions.
Sooo, unless you had a good idea of what version your previous save came from, you could end up overwriting variable values for events you've done, using a 'save fix mod tool' created specifically for this purpose. There may be a way to write a python expression or something to check for undefined variables, assigning a baseline value to such undefined variables, but python isn't my jam. It'd also be nice if all undefined variables could be assigned a 'base' value beforehand so that you would simply rely on this 'base' value (could be all undefined variables=false or all 0), but that's not how RenPy works currently...
Maybe someone here has seen/figured out a way to do this?